Many measurement situations arise in which it is desired to accurately display the waveform of an extremely short duration electrical event superimposed on a long time duration signal. An example of such a situation is the measurement of a response pulse having a width of less than 100 nanoseconds which occurs hundreds of microseconds after stimulus of some type, such as occur in radar, sonar, time-domain reflectometry, and certain network analyzer situations.
Attempted prior art solutions to this measurement problem have been unsatisfactory. Conventional delayed-sweep oscilloscopes simply do not have the resolution, triggering accuracy, or time-base stability for such precise measurements. It is essential that the response signal be synchronized with the stimulus signal and with the timing signal of the measuring circuit in order to provide signal acquisition with any kind of precision. Thus, in stimulus-response situations of the type mentioned above, it is desirable that the stimulus be generated in response to a start signal from the acquisition circuit's time base.
Conventional analog sampling techniques do not provide the high resolution required for faithful signal replication because of temporal inaccuracies resulting in a phenomena known as jitter--the smearing or slewing of displayed acquired data. Since jitter is typically expressed as a percentage of the overall time delay from the time base start to the point at which a sample is taken, that percentage may actually exceed the window of an extremely narrow pulse to be acquired a comparatively long time after stimulus, causing total destruction of any meaningful data.
A digital approach can aid the solution of this problem by allowing the use of a stable crystal-controlled clock timebase to act as a basis for generating the necessary timing signals. Such a time-base may have jitter percentages many orders of magnitude better than those obtainable with an analog system. However, a problem associated with such a digital timebase is that since the minimum sampling period is defined by the period of the clock pulse, extremely narrow pulses falling between successive sample points will be lost. For example, for a 100-megahertz clock, no better than one part in 10.sup.8, or 10 nanoseconds, can be resolved. Furthermore, the use of such a high-speed clock over the entire waveform requires the use of high-speed circuitry, not to mention the excess consumption of energy. Also, since memory space may be limited, another associated problem is deciding which samples to store and which ones to discard.